1. Field of the Invention
The invention relates to an electric power steering system.
2. Discussion of Background
There is a so-called rack assist-type electric power steering system (EPS). The conventional electric power steering system includes a hollow shaft through which a rack shaft is passed and which is rotated by a motor. The electric power steering system applies assist force to a steering system by converting the rotation of the hollow shaft into an axial movement of the rack shaft with the use of a ball screw device.
Generally, such a ball screw device is formed by arranging a plurality of balls in a spiral rolling passage that is defined by a thread groove formed in the outer periphery of the rack shaft and a thread groove formed in the inner periphery of a ball screw nut, the thread grooves facing each other. The balls are held between the rack shaft and the ball screw nut in the rolling passage. When the ball screw nut rotates relative to the rack shaft, the balls roll in the rolling passage upon reception of a load (friction force) from the ball screw nut and the rack shaft. The ball screw device has a return passage that connects two points set in the rolling passage to each other. The balls roll in the rolling passage, and are returned from the point set on the downstream side to the point set on the upstream side through the return passage. In this way, the ball screw device converts the rotation of the ball screw nut into an axial movement of the rack shaft by causing the balls that roll in the rolling passage to endlessly circulate through the return passage.
There is a so-called deflector-type ball screw device in which the above-described return passage is formed by attaching a circulation member (deflector) to fitting holes that radially pass through a ball screw nut (for example, see U.S. Patent Application Publication No. 2011/296939 A1). The circulation member has the function of scooping balls from a rolling passage and the function of ejecting the balls to the rolling passage. In many cases, the circulation member is fixed to the ball screw nut by press-fitting the circulation member into the fitting holes or by using an adhesive agent.
However, when the circulation member is fixedly press-fitted into the fitting holes, the circulation member slightly deforms due to a press-fitting load. Therefore, deformation of the return passage may occur and smooth rolling of the balls may be hindered. On the other hand, when the circulation member is fixed to the fitting holes by an adhesive agent, the adhesive agent may flow into the rolling passage or the return passage and smooth rolling of the balls may be hindered. As described above, with the conventional methods of fixing the circulation member, for example, abnormal noise may be generated due to hindrance to smooth rolling of the balls.
Japanese Patent Application Publication No. 11-270647 (JP 11-270647 A) describes an EPS in which a ball screw nut is fixed to the inner periphery of a motor shaft that serves as a hollow shaft, protrusions that contact the inner periphery of the motor shaft are formed on the outer surface of a circulation member, and the motor shaft supports the circulation member via the protrusions. However, the protrusions described in JP 11-270647 A are used to suppress backlash of the circulation member due to pressing force that is applied to the circulation member from a ball that is scooped from a rolling passage or a ball that is ejected to the rolling passage, and therefore the protrusions are formed in a linear shape so as to cross a return passage, that is, so as to cross the direction in which the return passage extends. That is, the protrusions are formed at positions that overlap with the return passage when viewed in the radial direction of a ball screw nut. Therefore, if external force acts on the protrusions from the radially outer side, for example, when the ball screw nut is fitted to the motor shaft, deformation of the return passage is likely to occur.